I've got a sneaking suspicion that she's gained immunity or at least resistance to the high levels of seither that would kill most other humans. When you lose to Nu in Arcade, she makes a comment along the lines of "termination of target possessing seither amp: complete". Furthermore, several context clues fit this theory. It's stated that seither concentration is higher at lower altitudes and thus the caste system of the hierarchical cities are designed with the poor farther down. The Kaka clan is known to be immune to these concentrations and their village is even farther down in altitude than the rest of Kagutsuchi. Litchi spends a sizable amount of time with the Kaka clan but has not been shown to suffer any ill effects.
A single winquote and a bit of circumstantial evidfence could be wrong but it fits into some things with her character. Seeking a way to neutralize seither (and possibly magic as a whole) will fit into Sector Seven's whole "FOR SCIENCE!" way of doing things. Seither immunity allows her to chase Arakune and possibly may have been part of her original plan for curing him. Also, as an actual doctor, seither immunity could simply be key to helping her treat those who have suffered ill effects from it taking into account the concept of "physician heal thyself".
